oli-widgets should take a whole feedback object

[robsimmons]

This will prevent us from having to use the currently pervasive ugly pattern

        if (data.parts[0].feedback) {
            $("#feedback").html(widgets.feedback(data.parts[0].feedback.message, data.parts[0].feedback.correct));
        } else {
            $("#feedback").html(null);
        }

and instead just allow writing

            $("#feedback").html(widgets.feedback(data.parts[0].feedback);